Standing too close is certainly one possibility. Another thing to make sure of is that you are still turning your shoulders and not simply using your hands in the backswing. Also, make sure you're not trying to flip or lift the ball with the club but still hitting downwards.

Off hard, short grass the bounce of a sand wedge does make it difficult to get a clean contact. Thin shots are certainly likely and if you overcompensate then a fat shot might result, but probably not a shank.